1. Field of the Invention
The present disclosure relates to a fixing apparatus which heats a toner image that has been transferred to a recording medium and thereby fixes the toner image onto the recording medium, and to an image forming apparatus using such a fixing apparatus.
2. Description of the Related Art
The basic constituent elements of an image forming apparatus such as a copying machine, a facsimile machine or a printer include: an image forming section which forms a toner image on an image carrying body (for example, a photosensitive drum); a transfer unit which transfers the toner image on the image carrying body to a sheet, which is one example of a recording medium; and a fixing apparatus which heats and fixes the toner image transferred to the sheet, onto the sheet.
A growing number of fixing apparatuses employ a belt system in which the heat capacity can be set to a low value with a view to shortening the warm-up time at apparatus start-up and reducing energy consumption, etc. Furthermore, attention has been drawn to electromagnetic induction heating (IH) systems, which are capable of very fast heating and high-efficiency heating. An electromagnetic induction heating system excites an induction current in a heating roller or fixing belt, by means of a magnetic flux generated by passing a high-frequency current through an induction coil, and uses the resistance of the actual heating roller or fixing roller itself to produce Joule heat in the heating roller or fixing belt (by induction heating). By means of this Joule heat, the toner image is fixed to a sheet (recording medium) in a nip section between the fixing roller (or fixing belt) and a pressurization roller. A fixing apparatus which combines an electromagnetic induction heating system and a belt system has been developed as a product.
A conventional fixing apparatus which combines an electromagnetic induction heating system and a belt system includes: a fixing belt; a pressurization roller which, together with the fixing belt, forms a nip section through which a sheet carrying a toner image is passed; a fixing roller and a heating roller about which the fixing belt is wrapped; and a coil unit, disposed in a position opposing the heating roller, which lets the fixing belt generate heat by induction heating.
The coil unit includes a plurality of cores which form magnetic paths along which the magnetic flux generated by the coil passes, and a magnetic shielding plate is installed on a center core of these cores. The position of the magnetic shielding plate is switched between a shielding position where the plate is disposed in the magnetic path and shields the magnetic flux, and a withdrawn position where the plate is withdrawn from the magnetic path and does not shield the magnetic flux, in accordance with the amount of rotation of the center core. By appropriately switching the position of the magnetic shielding plate between the shielding position and the withdrawn position in accordance with the size of the sheet which is passed through the nip section, overheating of the fixing belt outside the paper passage region, where the sheet is not in contact with the fixing belt, is suppressed.
However, although a conventional fixing apparatus is able to suppress overheating outside the paper passage region of the fixing belt, it is difficult to shorten the warm-up time of the fixing belt. Since the fixing roller and the heating roller which are wrapped about the fixing belt have a high heat capacity, then a large amount of heat is transferred from the fixing belt that generates heat by induction heating, to the fixing roller and the heating roller. If the amount of heat transfer is large, then the time required until the fixing belt is sufficiently heated increases.